It is known that polymers in which the primary structure is precisely controlled may be spontaneously assembled to form a higher-order structure. Specific examples thereof include structures such as micelles and vesicles. In the case of such a structure in which polymers are self-assembled, various types of molecules can be designed, and in addition to characteristics originally had by the polymers, new functions may be had by the structure. Utilization of such structures in which polymers are self-assembled has been examined in various fields such as those of the drug delivery system and material science.
For example, Japanese Laid-Open Patent Publication No. H08-188541 (Patent Document 1) discloses an electrostatically-bonded polymer micelle drug carrier made of a block copolymer having a non-charged segment and a charged segment. Further, WO2006/118260 (Patent Document 2) discloses, as an electrostatically-bonded vesicle, a vesicle composed of a membrane formed by interaction between two copolymers.
Further, Helmut Schlaad et al., Macromolecules, 2003, 36(5), 1417-1420 (Non-Patent Document 1) discloses that a vesicle called a polymersome can be formed using a block copolymer composed of a poly(1,2-butadiene) block and a poly(cesium methacrylate) block and a block copolymer composed of a polystyrene block and a poly(1-methyl-4-vinylpyridium iodide) block.
Though it is expected that a structure formed from a polymer material has excellent availability, it is often required to use an organic solvent such as chloroform at the time of the production. Moreover, at the time of the production of a structure in which a polymer material is self-assembled, complicated operation comprising a plurality of steps may be required. Therefore, it can be said that it is desired to create a structure having availability which can be produced by simple operation.
In Anraku Y. et al., J. Am. Chem. Soc., 2010, 132(5), 1631-1636 (Non-Patent Document 2), a vesicle made by self-assembly of a block copolymer having an electrically non-charged hydrophilic segment and a charged segment (e.g., polyethylene glycol (PEG)-polyanion) and a copolymer having an electric charge which is opposite to that of the charged segment (e.g., polycation) is disclosed by a part of the present inventors. According to this technique, only by mixing two types of polymer aqueous solutions, a vesicle made of one electrostatically-bonded membrane with a uniform diameter of 100 to 400 nm can be conveniently produced. It is usually considered that a vesicle obtained by self-assembly of polymers is allowed to include/carry various substances in its hollow and to release the included things under appropriate circumstances for use (for the outline, see: H. Nyin et al., Soft Matter, 2006, 2, 940-949 (Non-Patent Document 3); “Riposomu Ouyou no Shin-tenkai (New development of application of liposome)”, Kazunori Akiyoshi et al., ed., NTS, 2005 (Non-Patent Document 4); and Akihiro Kishimura and Kazunori Kataoka, “Jinko Ketsueki (Artificial blood)”, 2009, 17, 73-81 (Non-Patent Document 5)). Therefore, regarding the above-described electrostatically-bonded vesicle, it is desired to establish a convenient crosslinking method and a method of controlling the release property and stability of the vesicle in an environmentally-responsive manner.